JP-2002-13844-A, and JP-2000-81157-A having counterparts U.S. Pat. No. 6,189,326 and EP-0971184-B1 disclose pressure control valves that are preferably applicable to a vapor-compression refrigeration cycle for an air conditioning system for a vehicle.
As shown in FIG. 11, the pressure control valve according to JP-2002-13844-A is applied in a refrigeration cycle having a closed circuit that is composed of a compressor 1, a condenser (gas cooler) 2, a receiver (gas-liquid separator) 9, an expansion valve (pressure control valve) 103 and an evaporator 4, to circulate HFC-134a refrigerant therein. The pressure control valve 103 has a temperature sensing portion 130 that is formed by sandwiching a peripheral portion of a film-shaped diaphragm 131 between a first housing (cap member) 132 and a second housing (flange member) 133 so that the diaphragm 131 bulges in accordance with a variation of a pressure in a temperature sensing chamber S. Peripheral portions of these three members 131, 132, 133 are welded to each other, and the second housing 132 is fixed on a main body 134 of the pressure control valve 103. Above-mentioned construction of the temperature sensing portion 130 endures well in a case in which a refrigerant having a relatively low working pressure (e.g. HFC-134a) circulates, and the diaphragm 131 is subjected to a relatively low refrigerant pressure at an outlet side of the evaporator 4. In this regard, in a CO2 refrigeration cycle in which CO2 refrigerant circulates, it is necessary to keep the refrigerant pressure in a high pressure range in accordance with a refrigerant temperature at an outlet side of the gas cooler (radiator), in order to maximize a coefficient of performance (COP). If the pressure control valve 103 is applied in the CO2 refrigeration cycle, the diaphragm 131 is subjected to the high pressure at the outlet side of the gas cooler 2. The above-mentioned construction of the temperature sensing portion 130 does not endure this high refrigerant pressure, and the pressure control valve 103 is not suitable for a use in the CO2 refrigeration cycle.
As shown in FIG. 12, the pressure control valve according to JP-2000-81157-A (U.S. Pat. No. 6,189,326, EP-0971184-B1) is applied in a refrigeration cycle having a closed circuit that is composed of a compressor 1, a gas cooler (radiator) 2, a pressure control valve 203, an evaporator 4, an accumulator 5 and an internal heat exchanger 8, to circulate CO2 refrigerant therein. The pressure control valve 203 has a temperature sensing portion 230 that is formed by bending a peripheral portion of the diaphragm 231 generally into a right angle to provide a bent portion 231a as shown in FIG. 11, fitting the bent portion 231a of the diaphragm 231 between a diaphragm cover (cap member) 232 and a diaphragm support (flange member) 233, and welding these three members 231, 232, 233 at a rim portion of the bent portion 231a. 
However, a construction of the temperature sensing portion 230 of the pressure control valve 203 has the following disadvantages.
(1) The diaphragm 231 must be made of a material having a relatively high strength such as precipitation hardening stainless steel, in order to secure enough endurance for working. As mentioned above, CO2 refrigerant has a relatively high working pressure with respect to the working pressure of the conventional HFC-134a refrigerant. Thus, the pressure control valve 203 according to JP-2000-81157-A (U.S. Pat. No. 6,189,326, EP-0971184-B1) secures sufficiently large burst pressure, by using the diaphragm 231 having relatively large thickness and by bending the peripheral portion of the diaphragm 231 into an L-shape.
In this construction, however, the diaphragm 231, which has a relatively large strength, is bent, so that an elastic restoration of the diaphragm 231 distorts in a proximity of the bent portion 231a, to deteriorate flatness of the diaphragm 231. Thus, when the diaphragm 231 is fitted to the diaphragm cover 232 and to a diaphragm support 233, a gap is generated between the diaphragm cover 232 or the diaphragm support 233 and the diaphragm 231, to decrease a dimensional accuracy and/or an endurance of the diaphragm 231.
(2) CO2 refrigerant has a relatively high working pressure, to increase thickness of the diaphragm cover 232 and the diaphragm support 233. Further, the diaphragm cover 232 and the diaphragm support 233 are made of the same kind of stainless material as the diaphragm 231 is, to be welded to the diaphragm 231. Thus, the diaphragm cover 232 and the diaphragm support 233 are not easily machined, and the diaphragm cover 232 and the diaphragm support 233 having large thicknesses cannot be processed by stamping, to raise manufacturing cost thereof.(3) CO2 refrigerant is ordinarily used in supercritical fluid phase, so that a refrigerant pressure increases as a refrigerant temperature increases. When the compressor 1 is stopped, the CO2 refrigerant cooled in the gas cooler 2 does not flow especially in the temperature sensing portion 230 that is filled with the CO2 refrigerant, and the CO2 refrigerant in the temperature sensing portion 230 is heated up to a temperature in the engine room. Thus, the refrigerant pressure in the temperature sensing portion 230 exceeds a maximum working pressure of the refrigeration cycle. It is necessary to prevent parts of the pressure control valve 203 from scattering even if the refrigerant pressure increases beyond a strength of the temperature sensing portion 230. However, rim portions of the members 231, 232, 233 have relatively small thicknesses to secure enough melt depth, so that the strength of the temperature sensing portion 230 is small in the rim portions of the members 231, 232, 233 with respect to the strength in the other portion. Thus, a breakage of the temperature sensing portion 230 can start in the rim portions to scatter the diaphragm cover 232.